Moon's magnetic umbrellas may shield future spaceships

LUNAR watchers have been crying over spilt milk for decades. Now the mystery of milky splotches on the moon's surface might finally be solved - "mini magnetospheres" seem to be protecting the blobs from the scourge of the solar wind.

Ruth Bamford of the Rutherford Appleton Laboratory, UK, and colleagues have now shown how such magnetic fields might work, which could help spaceship builders devise magnetic shielding to protect astronauts from harmful radiation.

Unlike Earth, the moon has no enveloping magnetic field to fend off the constant flow of charged particles that make up the solar wind. These particles interact with lunar soil, darkening it over thousands of years, except for paler blobs that seemed to have no obvious source. "It very quickly became quite a mystery as to what it was," says Bamford.

Missions during the Apollo era matched the splotches to bubbles of magnetic field just a few hundred metres across, which scientists at the time thought might be deflecting solar particles.

Further measurements, though, showed the fields were so weak that protons from the solar wind ought to slip right through them.

In the new lab test, Bamford and colleagues shot a beam of protons and electrons - representing the solar wind - at a small magnet.

Sure enough, the charged particles parted over the magnet like rain flowing over an umbrella (Physical Review Letters, doi.org/h8k).

"As far as I know, this is the first time that anybody has done an actual experiment," says Georgiana Kramer of the Lunar and Planetary Institute in Houston, Texas.

The team thinks that negatively charged electrons follow the field lines of the mini magnetospheres, while the protons start to glide through. Then the separation between negative and positive charges creates an electric field inside the bubble that is stronger than the magnetic field. This pulls the protons back out so that both types of particle slide over the bubble.

Bamford says her team has been working on magnetic deflector shields for spacecraft. Critics suggest running them would take too much energy to be practical. But Bamford thinks the moon bubbles could inspire shields generated at lower energies.

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